Machine tools of this kind usually have a numeric control, e.g., an NC or CNC control. The control data, such as position and/or path or contour data for the desired workpiece machining needed for the numeric control, are supplied in the form of control inputs to the control device and are converted there into control signals for servo drives of the machine tool.
As one example for this, we mention an EDM machine, i.e., an erosive die sinking or cutting machine. These EDM machines are used, for example, for the manufacture of stamps and/or dies of conducting materials. The extremely high machining precision is an advantage. An economically rational usage of these EDM machines consists primarily in the production of molds for mass production. This yields the need during standard operation of the EDM machine to input the individual control data applicable to the particular workpiece, before the machining of one or several workpieces. In addition to the assignment of the desired machining steps, possible accidents must also be taken into account, e.g., a wire break during cutter eroding, i.e., situations during the machining in which the operator must intervene manually in the machining sequence. The control device thus stops the machining sequence until receipt of a Continue command from the operator, and prompts the operator to remove or to attach a "dropout piece," for example, such as is produced when cutting a die, or a stamp, to change a tool, etc. The following two process interruptions can thus be distinguished: the planned, predictable interruption, e.g., tool change, dropout piece fastening or removal, and the unpredictable interrupt, e.g., wire break, short circuit, etc.
When preparing the machine tools for a new machining sequence, the so-called set-up, the machine operator must thus program for the predictable interrupts or "breaks," when the machining sequence stops, and the machine awaits additional instructions from the operator. With increasing complexity of the workpiece machining on modern machine tools, e.g., three-dimensional eroding with an EDM machine, it will be increasingly more difficult for the machine operator to understand which processes have been already completed in the overall machining operation, and where the machining sequence is currently located.
Control methods of the aforesaid type corresponding to the state of the art require control inputs in the form of completed "programs," so-called sequential control programs. A program of this type includes all control data and specifies which cutting paths, for example, are to be performed with which quality at which location on the machined workpiece and in which sequence, etc. The control data are specified in a sequential series of commands. Thus, a command corresponding to a particular process step remains active until it is replaced by a new command for a new machining step in the command sequence of the control program. If an unforseeable accident occurs in this sequential machining, then the control device stops the entire machining operation sequence and waits for an input by the operator. For example, machinery running overnight can be stopped relatively early in the machining sequence in case of an accident. After correcting the accident, the part of the machining can only be continued on the following day, which is a disadvantage. Control devices of this type thus have undesirable down times and do not operate sufticiently economically.